Information is recorded on and read from a moving magnetic tape with an electromagnetic read/write head positioned next to the magnetic tape. The electromagnetic “head” may be a single electromagnetic head or, as is common, a series of read/write electromagnetic head elements stacked individually and/or in pairs within the head unit. Data is recorded in tracks on the magnetic tape by moving the magnetic tape lengthwise past the electromagnetic head. The electromagnetic head elements are selectively activated by electric currents representing the information to be recorded on the magnetic tape. The information is read from the magnetic tape by moving the magnetic tape longitudinally past the electromagnetic head elements. Magnetic flux patterns on the magnetic tape create electric signals in the electromagnetic head elements as the magnetic tape moves along. These electrical signals represent the information stored on the magnetic tape.
Data is recorded on or read from each of the parallel tracks on the magnetic tape by positioning the electromagnetic head elements at different locations across the magnetic tape. Electromagnetic head elements are moved from track to track, as necessary, either to record or to read the desired information. A head position actuator operatively coupled to servo control circuitry controls movement of the electromagnetic head according to servo information recorded on the magnetic tape. Tape drive head positioning actuators often include a lead screw driven by a stepper motor, a voice coil motor, or a combination of both. The head position actuator moves the electromagnetic head along a path perpendicular to the direction of travel of the magnetic tape. The electromagnetic head elements are positioned as close to the center of a track as possible based upon the servo information.
Two important considerations in the operation of tape drives are read errors and write errors. Read errors occur when data incorrectly is read from the magnetic tape; write errors occur when data is incorrectly written onto the magnetic tape. Tape storage protocols often include error-detection mechanisms that enable detection of a read error. When a read error is detected, the magnetic tape can be stopped and backspaced so that the data can be re-read. This view of read errors assumes that data has been stored on the magnetic tape correctly and that the error occurred during the read operation. If data was not stored on the magnetic tape correctly in the first place, then correcting the erroneously written data during the read process is very difficult. Write errors, therefore, are more serious than read errors. A simple write mechanism has no way to verify that the data has been correctly transferred to the magnetic tape.
Two contributors to read/write errors are shock and vibration that result from incidental, intended, or unintended motion of a tape transport mechanism in the tape drive during operation. The tape transport mechanism typically is housed in the body of the tape drive. Any shock and vibration applied to the body of the tape drive couples mechanically to the tape transport mechanism and therefore to the electromagnetic head. Such disturbances that occur during a write process can cause misalignment between the electromagnetic head and the tape track to which data is being written. This misalignment increases the chance that a write error will occur. Similar disturbances that occur during a read process also can cause read errors.
Because write errors are more serious than read errors, electromagnetic head assemblies often comprise a pair of elements, a “front” electromagnetic head and a “back” electromagnetic head, separate from each other, but closely spaced together. These assemblies operate by writing data to the magnetic tape with the front electromagnetic head and then immediately reading the just-written data from the magnetic tape with the back electromagnetic head. If the data read matches the data written, then the data on the magnetic tape is assumed to be correct. If the two versions of the data do not match, then corrective action can be taken. One form of corrective action writes a mark on the magnetic tape that identifies the errored data as invalid and then rewrites the data onto a different part of the magnetic tape.
Unfortunately, the write-read technique can be fooled by shock and vibration applied to the body of the tape drive. The write and read electromagnetic heads employed in the write-read method are rigidly connected to each other and undergo essentially identical motion in response to any shock or vibration that reaches the tape transport mechanism that supports the electromagnetic head. Therefore, if shock or vibration causes the front electromagnetic head to write data to an incorrect location on the magnetic tape, then the confirming data read by the back electromagnetic head comes from the same incorrect location, and no error is detected. Upon playback of the same data, however, a read error may occur because the read electromagnetic head may not be aligned to the same position on the magnetic tape as when the data was written in the presence of shock or vibration. Even in the absence of a write error, shock or vibration that occur during a read operation can cause momentary misalignment of the electromagnetic head and the magnetic tape, thus causing a read error.
To summarize, shock and vibration during a write operation can lead to data that is erroneously written onto magnetic tape, and the resulting errors may not be detected by the write-read technique. Shock and vibration that occur during a read operation can lead to read errors regardless of the quality of the data on the magnetic tape.